1. Field of the Invention
The present invention relates to an electrophotographic photosensitive element having on a conductive support thereof a photosensitive layer composed in combination of a carrier-generating phase with a carrier-transporting phase.
2. Description of the Prior Art
There have been widely known electrophotographic photosensitive elements which have a photosensitive layer comprised principally of such inorganic photoconductive materials as selenium, zinc oxide, cadmium sulfide and the like. However, these elements cannot necessarily be satisfactory in respect of the stability to heat and durability, and besides have problems in the handling and the manufacture due to their toxic character.
On the other hand, electrophotographic photosensitive elements having a photosensitive layer comprised principally of organic photoconductive compounds have in recent years focused the spotlight of attention upon such various advantages thereof as the relative ease of manufacture, inexpensiveness, ease of handling and the like, and in addition they are generally superior in the stability to heat to those having selenium as the sensitive element thereof. Among such organic photoconductive compounds, poly-N-vinyl carbazole is best known. There has already been made practical reality an electrophotographic photosensitive element having a photosensitive layer comprised mainly of a charge-transfer complex formed from said poly-N-vinyl carbazole and a Lewis acid such as 2,4,7-trinitro-9-fluorenone, while there has been known an electrophotographic photosensitive element having a function-separated photosensitive layer of either the multilayered type or dispersed type constituted so that carrier-generating function and carrier-transporting function may be performed in different material phases respectively; for example, an electrophotographic photosensitive element having a photosensitive layer composed in combination of a carrier-generating layer of amorphous selenium thin layer with a carrier-transporting layer of poly-N-vinyl carbazole is now available for practical use.
However, a layer comprising poly-N-vinyl carbazole has so little flexibility that said layer is hard and fragile, tending to cause cracks and peeling off of the layer, thereby causing the photosensitive element to become less durable, while when a plasticizer is added to improve such defects, the residual electric potential at the time when provided in the electrophotographic process becomes high, and as the element is repeatedly used, the residual electric potential becomes accumulated, thereby gradually producing fog on the reproduced image.
In addition, low molecular weight organic photoconductive compounds are desirable in that they, for lack of film formability, are generally used in combination with a binder, so that, by selecting the kind and composition ratio of binders to be used, the physical or electrophotographic characteristics thereof may be controlled to some extent. However, the number of kinds of organic photoconductive compounds having high compatibility with binders is limited as understood from the fact that only a few are applied to the composition of photosensitive layers for electrophotographic photosensitive element.
For example, the 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole as described in U.S. Pat. No. 3,189,447 has low compatibility with binder that is in general suitably used as materials for the photosensitive layer of an electrophotographic photosensitive element, so that when the material is used to form a photosensitive layer by mixing with such a binder as polyester, polycarbonate in the proportion required for obtaining desirable electrophotographic characteristics, crystals of the oxadiazole come to deposit at a temperature of above 50.degree. C., thereby resulting in the deterioration of electrophotographic characteristics such as the charge acceptance, photosensitivity, etc., while the diaryl alkane derivatives described in U.S. Pat. No. 3,820,989 have no problems in respect of the compatibility with binders, but have little stability to light, so that when applied to the composition of the photosensitive layer of an electrophotographic photosensitive element intended for use in the repetitive transfer type electrophotographic process in which charging-exposure cycle is repeatedly effected, the photosensitivity of the photosensitive layer becomes gradually lowered.
The p-diethylaminobenzaldehyde-diphenylene hydrazone described in Japanese Patent Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 81847/1979 has good compatibility with binder and enables to provide an electrophotographic photosensitive element having relatively good initial characteristics, but said photosensitive element is lacking in durability because as it is repeatedly used, its characteristics such as the sensitivity and the residual potential show a change, and in the concrete the sensitivity becomes gradually lowered and the residual potential becomes accumulated.
As has been mentioned, it is the fact that there are still not found any organic photoconductive compounds having practically suitable characteristics for an electrophotographic photosensitive element.